Allelic and isotypic (haplotype) exclusion establishes that each B cell expresses only one immunoglobulin heavy and light chain pair and, consequently, exhibits a single specificity. Nevertheless, B cells expressing either two heavy or two light chains (haplotype-included) exist in healthy individuals and wild-type mice at a frequency estimated to be approximately 2% of the B cell population. Analyses of immunoglobulin transgenic mice have demonstrated that autoreactive B cells can bypass mechanisms of central tolerance and develop into mature B cells by co-expressing non-autoreactive antigen receptors. Thus, these cells are haplotypeincluded and co-express an autoreactive and non-autoreactive antigen receptor. Because the majority of primary immunoglobulin gene rearrangements have been shown to generate an autoreactive specificity, we propose that most haplotype-included B cells in wild-type mice are autoreactive although the specificity of these cells in mice or humans has not been investigated. The goal of this proposal is to understand the nature and regulation of haplotype-included B cells in mice and to determine if these cells are associated with autoimmunity. To accomplish this we outline three Specific Aims that will characterize haplotypeincluded B cells in mice expressing a wild-type antibody repertoire, determine whether these B cells contribute to the autoimmune process of autoimmune-prone mice, and to understand how haplotypeincluded B cells are physiologically regulated. Haplotype-included autoreactive B cells will be evaluated both in vitro and in vivo using various mouse models and a variety of techniques that include the generation and characterization of B cell hybridomas and the use of cell adoptive transfer. Because autoimmunity is a significant health issue that has multiple, as of yet, poorly understood etiological bases, a molecular and cellular analysis of haplotype-included B lymphocytes is warranted. In particular, we predict haplotypeincluded B cells to be unique in that the autoantibody they express have the potential to exhibit a high avidity interaction with autoantigen and produce antibodies that simultaneously bind foreign as well as self-antigens.